In general, SUJ2 (JIS G4805) which is high-carbon chromium bearing steel or the like is widely employed as the material for a rolling contact member constituting a rolling bearing. In a case of manufacturing a rolling contact member with a material such as a steel bar made of high-carbon steel such as SUJ2, this material is hot-worked, thereafter subjected to spheroidizing annealing, further cold-worked, and subjected to quench hardening in general.
An example of a conventional process for manufacturing a rolling contact member and a rolling bearing is now described. Referring to FIG. 11, a material such as a steel bar made of high-carbon steel such as SUJ2 is prepared in a step (S111). Then, the material is cut in a step (S112), and the cut material is hot-forged in air in a step (S113) so that a blank ring is prepared. Thereafter the blank ring is heated in air to a prescribed temperature in a step (S114A), to be subjected to spheroidizing annealing. Thus, the blank ring is softened while the state of the microstructure of the steel constituting the blank ring is improved, and workability (easiness of working) is improved. This blank ring is cold-forged in a step (S116), and a stepped ring for collecting an inner ring and an outer ring of the rolling bearing is prepared. Then, this stepped ring is separated into an inner ring portion and an outer ring portion in a step (S117), and the overall surfaces of the inner ring portion and the outer ring portion are turned in a step (S118A). Thus, formed rings having schematic shapes of the inner ring and the outer ring of the rolling bearing are prepared.
Further, the formed rings are quench-hardened in a step (S119), thereafter subjected to tempering in a step (S120), and subjected to finishing such as grinding in a step (S121). Thus, the inner ring and the outer ring of the rolling bearing as rolling contact members are completed. In a step (S122), the inner ring and the outer ring and separately prepared rolling elements etc. are combined with each other, to complete the rolling bearing.
The hot forging in the step (S113) and the annealing in the step (S114A) are executed by heating the blank ring in air. Therefore, a layer of iron oxide (scale) is formed on the surface layer portion of the blank ring, while a decarburized layer reduced in carbon content as compared with the interior is formed immediately under the scale. At this time, the surface of the blank ring is remarkably irregularized due to the formation of the scale. In the cold forging in the step (S116), therefore, such a phenomenon (burring) may take place that the surface of the blank ring is internally rolled. If the burring takes place, this portion results in defects of the completed rolling contact members, and exerts bad influence on durability. If the carbon content in the surface layer portion is reduced due to the formation of the decarburized layer, not only sufficient hardness cannot be ensured by the quench hardening in the step (S119), but also tensile stress may remain in the surface layer portion, to exert bad influence on the durability.
In the step (S118A), therefore, the overall surfaces of the separated portions of the stepped ring are so turned that the scale, and the burr and the decarburized layer attributed to the scale are removed, and the bad influence on the durability of the rolling contact member attributed to the aforementioned scale and the formation of the decarburized layer is avoided. In the aforementioned manufacturing step, however, the overall surfaces of the separated portions of the stepped ring are so turned that not only the number of steps of working is increased but also the yield of the material is reduced, and the manufacturing cost for the rolling contact members and the rolling bearing is increased.
Another example of a conventional process for manufacturing a rolling contact member and a rolling bearing is now described. Referring to FIG. 12, this example of the conventional process for manufacturing a rolling contact member and a rolling bearing is basically similar to the conventional process for manufacturing a rolling contact member and a rolling bearing described with reference to FIG. 11. In the manufacturing process of FIG. 12, however, recarburization annealing of performing annealing while recarburizing the decarburized layer formed in the step (S113) is executed as a step (S114B) by controlling a carbon potential (Cp) value in the annealing atmosphere, in place of the annealing in air in the step (S114A) of the manufacturing process shown in FIG. 11. Thus, the decarburized layer disappears in the step (S114B), whereby the aforementioned full turning for the purpose of removing the decarburized layer is unnecessary. A step (S118B) of turning only an area hard to form by cold forging for such a reason that the shape thereof is complicated is executed in place of the step (S118A) of FIG. 11. Consequently, regions subjected to the turning can be reduced in the separated portions of the stepped ring, and the manufacturing cost can be reduced. In addition, various measures for suppressing formation of the decarburized layer or eliminating the formed decarburized layer are proposed (Japanese Patent Laying-Open No. 2002-285233 (Patent Document 1), Japanese Patent Laying-Open No. 2003-194072 (Patent Document 2), Japanese Patent Laying-Open No. 9-176740 (Patent Document 3) and Japanese Patent Laying-Open No. 11-347673 (Patent Document 4)).    Patent Document 1: Japanese Patent Laying-Open No. 2002-285233    Patent Document 2: Japanese Patent Laying-Open No. 2003-194072    Patent Document 3: Japanese Patent Laying-Open No. 9-176740    Patent Document 4: Japanese Patent Laying-Open No. 11-347673